Scientists have predicted that the hole in the ozone layer above Antarctica could be healed within 70 years. The prediction comes days after the hole reached its maximum size for this year, breaking previous records for late September.

The ozone "hole" opens up over the Antarctic every year in mid-August and usually peaks in size by September. It is not a hole in the literal sense because ozone still exists over the continent but significant amounts of the gas are destroyed in this area because of the temperature and presence of damaging gases such as chlorofluorocarbons (CFCs).

"This year it's been very cold in the Antarctic ozone layer and, as a consequence, [the hole] has had a chance to expand to quite a large size," said Jonathan Shanklin, head of the meteorology and ozone monitoring unit at the British Antarctic Survey.

At its peak last week the hole reached 10.4m square miles, just shy of the overall record of 10.8m square miles in 2000.

"That's larger than the entire continent of North America," said Paul Newman of Nasa's Goddard Space Flight Center in Maryland.

"The Antarctic ozone hole will reach sizes on the order of 8m-10m square miles nearly every year until about 2018 or so," he said. "Around 2018 things should slowly start improving, and somewhere between 2020 and 2025 we'll be able to detect that the ozone hole is actually beginning to decrease in size."

A report by the World Meteorological Organisation this month predicted a similar pattern.